1. Field of the Invention
The invention relates generally to communication systems and more particularly to a communication system having hubs and mobile units using dedicated and operational time slots where a mobile unit receives selection signals from the hubs during the dedicated time slots for selecting a particular hub and then communicates with that hub during the operational time slots.
2. Description of the Prior Art
Many wireless communication systems have been proposed or are in use where mobile transceivers communicate with hub transceivers. A commonly known example of such system is a cellular telephone system having mobile cellular phone transceivers in communication with hub or cell site transceivers. The cell site transceivers for the cellular telephone systems are geographically spaced at fixed locations with minimum overlap for signal coverage for populated areas throughout the world. When more than one cell site transceiver is within range of a mobile cellular phone, the system decides which cell site communicates with the mobile phone based at least partially on the signal strength from the mobile phone at each of the in-range cell sites. A great deal of inventive effort and elaborate decision making software is used by the cell sites for determining the particular cell site and particular channel of the cell site transceiver for communicating with a particular mobile transceiver and when the mobile transceiver is to be handed off to another cell site or channel. Obviously, given the popularity of cellular phones, such systems work well for their primary application of allowing individuals to use the worldwide telephone system to talk to each other while at least one of the individuals is mobile. However, cellular phone systems are optimized for continuous voice communications as opposed to data burst communication. Also, cellular phone systems are not optimum for applications where only a few cell sites or hub transceivers are required such as a communication system at a construction or mining site for communicating data in bursts with mobile transceivers on mobile machinery such as bulldozers, shovels, cranes, dump trucks, and the like. One limitation of existing cellular systems for such applications is that the cell sites require relatively expensive computing power in order to run the decision making software for handing off mobile transceivers from one hub to another. Another limitation for applications having many mobile machine units that need to transmit occasional data bursts is that relatively few signals from each mobile transceiver are available to the hubs for measuring signal strength from that mobile transceiver. Another limitation is that the cell sites in cellular telephone systems are deliberately located in order to minimize signal overlap, thereby minimizing the opportunity for using redundant transmission for improving the reliability of especially selected important message information that is generally needed by the mobile machine units by transmitting that information from more than one hub to each of the mobile units.
It is therefore an object of the present invention to provide a communication system where certain message information is transmitted from each of several hubs in order to improve the reliability of its reception by mobile transceivers.
Another object of the present invention is for a mobile transceiver to use selection signals in dedicated time slots from hubs in order to select a particular one of the hubs with which to communicate.
Briefly, in a preferred embodiment, the communication system of the present invention includes at least two hubs for transmitting selection signals and receiving and transmitting operational signals; and at least one mobile unit for receiving the selection signals for selecting a particular one of the hubs and then communicating with the selected hub with the operational signals. The system organizes time into non-overlapping time slots having dedicated time slots for the selection signals and operational time slots for the operational signals. Each hub transmits a selection signal during a dedicated time slot that is allocated to the selection signal from that hub. The mobile units receive the selection signals during the dedicated time slots and select the particular hub with which to communicate based upon data error rates in the respective selection signals. Each of the hubs has a channel for transmitting its selection signal and receiving and transmitting operational signals. The mobile units tune to the respective channels for receiving the selection signals during the dedicated time slots and then communicating with the selected hub with the operational signals during the operational time slots. Each of the selection signals from each of the hubs includes certain message data that is the same in all of the selection signals in order to increase the reliability of the mobile units receiving that data. Preferably, the certain message data includes precise positioning global positioning system (GPS) information for enabling mobile units equipped with GPS receivers to determine their geographical locations. Of course, the use of a dedicated time slot for a selection signal for each hub imposes time overhead on the system. However, for systems having only a few hubs, the present invention has several advantages over other systems.
An advantage of the present invention is that the reliability of communicating selected message data from the hubs to the mobile units is improved by transmitting the message data in the selection signals from all of the hubs.
Another advantage of the present invention is that every hub provides a selection signal at frequent intervals, thereby providing a mobile unit with frequent information upon which to select a communication link with a particular hub.
Another advantage of the present invention is that mobile units can select a particular hub without the need of transmissions from the mobile unit to the hub.
Another advantage of the present invention is that processing power for selecting hubs is distributed among mobile units, thereby reducing the processing power required in the hubs.